The casimiroa or white sapote (Casimiroa edulis) belongs to the Rutaceae family (the same as Citrus), but the fruit and tree bear little or no resemblance to citrus. The fruit which is very sweet and fine textured, is grown commercially in Mexico and California, but is presently almost unknown in Australia. It is native to the highlands of Mexico and Central America. Recent interest in the fruit for commercial production and possible export has prompted the need to assess cultivars, investigate flowering and pollination behaviour and develop management techniques.
CLIMATIC AND SOIL REQUIREMENTS
The Casimiroa is distinctly subtropical in its climatic requirements. Trees in California usually survive temperatures as low as minus 4°C. Flowering in S.E. Queensland can occur in the coolest winter months of July and August.
The trees are capable of growing in a wide range of soil types from sands to clay loams and do not appear to be susceptible to Phytophthora cinnamomi root disease. Soil pH should be maintained between 5.5 and 6.5.
Clean, air-dried seed will give reasonable germination percentage, up to three weeks after removal from the fruit.
Seed, raised in a suitable seed-raising mix or light porous soil, will germinate in 3 to 4 weeks in warm weather or with artificial heat (30°C) in cold weather.
Seedlings of C. edulis are recommended as rootstock material because of their hardiness to cold. Cultivars of C. tetrameria can be propagated successfully on C. edulis seedlings, although there is a tendency towards the overgrowth of the rootstock at the union in this combination.
Seedlings grow quickly and can be T-budded or chip budded in spring or autumn when stem diameter is about 10mm. Budwood that has matured and has grey bark should be used. The bud should be left exposed after tying, and after 3 to 4 weeks, if the bud has taken, the stock should be cut back to between 2 and 8 cm above the bud.
Casimiroa does not come true-to-type from seed, and trees are very variable. Seed does not store well (up to two months maximum), but a high percentage germination of seed, freshly removed from fruit can be expected.
Seedlings can be grafted whenever it is warm enough for the trees to grow well. Success rates, using budding and grafting, are often as high as 100 percent.
FRUIT QUALITY CHARACTERISTICS
The Casimiroa is currently being assessed in a wide range of climatic regions from Sydney to Cairns. Forty cultivars are under assessment at the Maroochy Horticultural Research Station at Nambour.
Cultivars are highly variable in their fruit quality characteristics; taste panels have described flavours of different cultivars as being similar to that found in pears, mango, custard apple and bananas! Flavour characteristics appear to be enhanced as fruit sugar levels increase during the season. For this reason, harvesting at the optimum time of maturity is important. With some cultivars this is relatively easy as there is a change in skin colour from green to yellow with advancing maturity. Many cultivars which have desirable eating qualities may not be suitable for commercial production because they are also highly susceptible to bruising. Many cultivars also exhibit a poor shelf life.
Although there are over 60 named cultivars of casimiroa in California and Australia, only a few will be acceptable for commercial production. Of the cultivars tested to date in Queensland, only three, Reinikie Commercial, Golden Globe, and Lemon Gold, can be tentatively recommended for commercial plantings. Of these, 'Reinikie Commercial' has the most attractive appearance. Further information is needed on yields and maturity time before definite recommendations can be made.
Reinikie Commercial. A large, flat fruit with a strong flavour. Maturity can be easily judged by a change in skin colour from green to yellow. Post-harvest shelf life is fair, but it is moderately susceptible to bruising. This cultivar has produced heavy crops on 3-year-old trees at Nambour and Rockhampton.
Golden Globe. A small, round, symmetrical fruit with excellent flavour. Maturity can be easily judged by a change in skin colour from green to pale yellow. Post-harvest shelf life is fair and it is moderately susceptible to bruising. Has produced moderate crops on 3-year-old trees at Nambour.
Lemon Gold. A moderately-sized fruit with a mild flavour and excellent texture. Post-harvest shelf life is good and susceptibility to bruising low. This cultivar has produced moderate to heavy crops at Rockhampton, but reports from California suggest that it may be a poor cropper under cooler growing conditions.
Other promising cultivars which require further testing are Candy, Rainbow, and Sheffler.
Planting. Planting distances of 8m between rows and 4m within rows is suggested. Young trees should be protected from frost.
Pruning. Cultivars differ markedly in their growth habits, with many producing strong apically-dominant growth. With the more vigorous, upright habit types, light regular pruning every 2 to 3 months for the first 3 to 4 years after planting is necessary. Light, regular pruning produces more fruiting laterals and a leafier canopy, which reduces sunburning of the fruit.
Thinning. Cultivars which naturally set heavy crops of small-sized fruit may need to be thinned to only one or two fruit per cluster.
Windbreaks. Young trees after planting, are particularly susceptible to wind damage and may need to be staked. Fruit is also easily blemished from leaf rub. The establishment of both major and minor windbreaks prior to planting is essential.
Irrigation. Young trees require frequent watering to ensure good growth. Once trees reach maturity, water stressing in late Autumn and Winter may improve flora initiation and encourage uniform flowering. Water stress during the flowering and fruit development periods, however, appears to be detrimental.
FLOWERING AND POLLINATION
Flowering occurs from late Autumn to early Summer. The flower clusters are produced either terminally or in the axils of mature leaves. Trees appear to respond to out-of-season floral induction by branch cincturing or water stress. Most commercial cultivars produce only functional female flowers due to the absence of pollen in the stamens, thus requiring other cultivars planted nearby to ensure pollination. Until further investigations can be carried out, it is recommended that both these cultivars be planted in blocks of 1 pollinator tree to every 9 trees of the commercial cultivar.
PESTS AND DISEASES
The most important insect pests are fruit fly, fruit spotting bug, soft brown scale, and mealybugs.
FRUIT MATURITY AND HARVESTING
In S.E. Queensland, fruit matures between early November and late January. In warmer subtropical areas, fruit mature up to one month earlier, while under cooler subtropical conditions, maturity may be delayed by 2 to 3 months.
Commercial development of this fruit on a small to medium scale appears good. The development of export markets to S. E. Asia may be feasible since the Asians like sweet fruit. Due to the highly perishable nature of this fruit, more information is needed on its post-harvest handling characteristics.
DATE: January 1987
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